In recent years, energy saving has been considered to be an important technical issue even in electrophotographic devices, and a drastic reduction in the amount of heat required by a fixing apparatus has been desired. Accordingly, the need for so-called “low-temperature fixability” in a toner, in which fixing with lower energy is possible, has been increasing.
Conventionally, a technique involving increasing the sharp melt properties of a binder resin is known as an effective method to enable the fixing at lower temperatures. In this point, polyester resins exhibit excellent characteristics.
On the other hand, from the viewpoint of improving image quality, reduction of the toner particle size and sharpening of the toner particle size distribution have progressed for the purpose of attaining higher resolution and higher definition. In addition, a spherical toner is now suitably used for the purpose of improving transfer efficiency and fluidity. As a method for efficiently preparing spherical toner particles with small particle sizes, a wet method is now used.
As a wet method capable of using a sharp-melting polyester resin, a “solution suspension” method has been proposed, in which spherical toner particles are produced by dissolving a resin component in an organic solvent which is immiscible with water, and dispersing the resultant solution in an aqueous phase to thereby form an oil droplet (Japanese Patent Application Laid-Open No. H08-248680). According to this method, a spherical toner with a small particle size can be easily obtained, which uses polyester having excellent low-temperature fixability as a binder resin.
Further, for the toner particles produced by the above-described solution suspension method using polyester as a binder resin, capsule type toner particles have also been proposed for the purpose of attaining even further low-temperature fixability.
Japanese Patent Application Laid-Open No. H05-297622 proposes a method in which a polyester resin, a low-molecular weight compound having an isocyanate group and the like are dissolved or dispersed in ethyl acetate to prepare an oil phase. This oil phase is dispersed in water to form droplets, and then interfacial polymerization of the compound having the isocyanate group is carried out at the droplet interface.
In this method, capsule toner particles having polyurethane or polyurea as an outermost shell can be obtained.
In addition, Japanese Patent Application Laid-Open No. 2004-226572 and Japanese Patent Application Laid-Open No. 2004-271919 propose a method in which toner base particles are prepared by a solution suspension method in the presence of resin fine particles formed from any one, or a combination thereof, of a vinyl resin, polyurethane resin, epoxy resin, and polyester resin to prepare toner particles having a toner base particle surface covered with the above-described resin fine particles.
Japanese Patent No. 3,455,523 proposes toner particles obtained by a solution suspension method using urethane-modified polyester resin fine particles as a dispersant.
International Publication No. WO2005/073287 proposes core-shell type toner particles formed by a shell layer (P) having one or more film-like layers formed from a polyurethane resin (a), and a core layer (Q) having one layer formed from a resin (b).
In these core-shell type toner particles, the viscosity of the core portion is lowered and the poor heat-resistant storage stability of the core portion is compensated with the heat-resistant storage stability of a shell portion. In this case, since a substance which is relatively strong against heat is used as the shell part, it is necessary to highly cross-link the resin used as the shell part or to use the resin having a high molecular weight. Consequently, there is a tendency for low-temperature fixability to be inhibited.
On the other hand, the coloring power of the toner is increased and the consumed amount of the toner is decreased by increasing the content of the colorant in the toner and controlling the dispersion state of the colorant. By decreasing the consumed amount of the toner, in line images and character images, a high-quality image can be provided with little scattering. Further, on the paper sheet, uneven portions are reduced and gloss is more uniform. Moreover, due to the decreased consumed amount of the toner, the toner container and the electrophotographic apparatus can be made more compact. In addition, running costs can be reduced, and power consumption can be decreased.
However, if the content of the colorant in the toner is simply increased, due to dispersion defects, for a color toner, the color gamut tends to be narrow and fixing impediments tend to occur as a result of the toner hardness increasing due to the filler effect. Further, due to a large amount of colorant being on the toner surface, the two-component developer carrier and sleeve parts tend to become contaminated. Moreover, when the moisture absorption properties of the colorant on the surface are large, a difference tends to occur in the charge amount depending on the environment.
Accordingly, in Japanese Patent Application Laid-Open No. 2006-206848, a toner particle is proposed which has a core-shell structure formed by a solution suspension method using a polyester resin in a binder resin and a polyester-containing urethane resin fine particle in the shell layer. Although exposure of the colorant can be thought to be suppressed by the shell layer, charge stability was insufficient. The reason for this can be thought to be due to a large amount of sulfonic acid groups and carboxyl groups introduced into the polyester-containing urethane resin fine particle, and the toner particle having a high amount of water absorption. When the toner particle has a high amount of water absorption, the charge amount of the toner particle tends to be insufficient. Further, the charge amount of a once-charged toner tends to decrease, and such a toner can have poor developing stability. In addition, the toner is plasticized by water absorption, and can agglomerate. This agglomeration tends to occur more easily when the glass transition temperature of a toner in a water-absorbed state decreases below a storage temperature.
Further, for a black toner, carbon black is usually used for the colorant. However, when the carbon black content is increased, a decrease in the charge amount, scattering and fogging during developing, and transfer defects tend to occur due to a decrease in resistance.